Deadline reports that Johnson will play Lindsey Cyr, the girlfriend of Boston mobster "Whitey" Bulger (Depp) and the mother of his only known child, a son named Douglas who died at the age of 6.

Bulger had two other notable girlfriends -- Teresa Stanley and Catherine Greig -- during his reign as the boss of the Boston Irish mob and a top echelon FBI informant, but those roles have yet to be cast.

Johnson joins Depp, Joel Edgerton and Guy Pearce in the Warner Bros. release, which is being rewritten and directed by Out of the Furnace's Scott Cooper. Production is expected to begin later this spring in Boston.

Dakota Johnson is poised to gain much notice for her role as "Anastasia Steele" in the forthcoming film version of the erotic best seller Fifty Shades of Grey.